1. Field of the Invention
The present invention relates to an optical free-space communication apparatus for performing communication by propagating an optical beam in free space.
2. Description of the Related Art
Typically, mobile communication by virtue of optical free-space communication is performed by exchanging optical beams B1 and B2, with signals superimposed thereon, between a mobile station 1 such as a vehicle and a fixed station 2 placed on the roadside, as illustrated by way of example in FIG. 1. It is necessary for the fixed station 2 and the mobile station 1 to have a function for performing an acquisition operation so that communication may be initiated when a mobile entity enters a certain communication area, and a tracking operation so that the communication may continue after the acquisition operation.
This is feasible by, for example, an apparatus illustrated in FIG. 2. In a transmitter unit of the apparatus, a primary signal containing information for communication is input from an input terminal 3, and is amplified by an amplifier 4 to an appropriate level. Meanwhile, an auxiliary signal for angle detection (hereinafter referred to as “pilot signal”) is generated by an oscillator 5. The primary signal and the pilot signal are combined by a combiner 6 to drive a light-emitting device 7 such as a semiconductor laser source or a light-emitting diode to convert the resulting signal into an optical beam. The optical beam is then emitted by a transmitting optical system 8 to a second party's apparatus as a transmission signal beam B3 having an appropriate angle of divergence.
In a receiver unit, an optical beam B4 from the second party's apparatus enters a receiving optical system 9 for primary signals and a receiving optical system 10 for pilot signals. The beams emerging from the receiving optical systems 9 and 10 are condensed onto a light-receiving device 11 for optical signals, such as an avalanche photodiode (APD) or a PIN diode, and a light-receiving device 12 for pilot signals which serves as an angle detection device, respectively. The primary signal received by the light-receiving device 11 for optical signals is converted from an optical signal to an electrical signal, and is then output from a primary signal output terminal 14 through an amplifier 13.
The light-receiving device 12 is, for example, a photodiode which is divided into four parts 12a to 12d, as shown in FIG. 3, such that outputs of the parts 12a to 12d on which a condensed optical spot S is formed are compared to determine the angle of the optical beam B4 incident on the receiving optical system 10 from the second party's apparatus. The angle signal from the light-receiving device 12 is arithmetically operated on by an arithmetic operation unit 15 to apply a driving signal to a vertical driving unit 16 and a horizontal driving unit 17 so that the optical spot S reaches the center of the angle detection device 12 so that there is no difference in angle between the received optical beam B4 and the receiving optical system 10. It is conditioned so that the optical axes of the transmitting optical system 8 and the receiving optical system 9 are placed at an identical angle, and as a result the transmission optical beam B3 is directed toward the second party's apparatus. In this way, a tracking operation is performed between the mobile station 1 and the fixed station 2.
In such mobile optical communication as illustrated in FIG. 1, the angle of divergence of the outgoing optical beam B1 should be increased as much as possible in order to ensure the acquisition operation. As the angle of divergence increases, however, the amount of light received by the second party's apparatus is reduced in inverse proportion to the square of the angle of divergence, and therefore reception may be difficult.
For example, as shown in FIG. 4, when the mobile station 1 is a vehicle that travels along a wide passageway such as a three-lane road, the divergence of optical beams B1a and B2a as indicated by dotted lines in FIG. 4 is necessary to ensure an optical acquisition at distance L. However, such divergence would reduce the light intensity per unit area in the receiver unit. Should the divergence of optical beams B1b and B2b as indicated by solid lines in FIG. 4 be necessary to provide a sufficient intensity of received light, optical acquisition would be in turn difficult.
To address the foregoing problems, one conceivable method is to prepare two types of transmission optical beams, namely, acquisition/tracking optical beams B1a and B2a containing only pilot signals, and communication optical beams B1b and B2b containing only primary signals. This approach is feasible because the pilot signal contained in an acquisition/tracking optical beam is usually a signal having a single frequency or a signal having an extremely narrow bandwidth compared to the primary signal, which can be then received at a high signal-to-noise ratio even if the received signal is small.
However, two types of signals, namely, an acquisition/tracking optical beam and a communication optical beam require two sets of the light-emitting device 7 and the transmitting optical system 8 shown in FIG. 2, leading to increased cost and size. In particular, it would be disadvantageous to dispose a larger apparatus on the mobile station 1, and, although an apparatus of this type needs to be rapidly driven, increased weight due to increased size would make it difficult.